In the BMA method for the manufacture of hydrogen cyanide, a gaseous mixture of methane and ammonia is reacted by passing it through suspended reaction tubes whose inner surfaces are covered with a layer of noble metal, preferably platinum and aluminum, at temperatures in the range of approximately 1000.degree. to 1350.degree. C. See, e.g., Ullmann, "Enzyklopadie der technischen Chemie" [Encyclopedia of Industrial Chemistry], 4th edition, vol. 9, page 659, (1975).
German Patent DE-PS No. 10 68 681 describes a modification of this method in which a non-metallic contact mass such as aluminum oxide is used. This method provides advantageous yields of hydrogen cyanide at the start of the reaction. However, in the course of the reaction, aluminum nitride is formed and the catalytic activity of the aluminum oxide diminishes sharply.
However, a coating of noble metal, e.g. platinum, deposited on the surface of the aluminum oxide, should neutralize this influence of the aluminum nitride.
The formation of aluminum nitride during the usage of .alpha.-aluminum oxide was evaluated more favorably in later methods, e.g. that of EP No. 0,088,988; however, an addition of sulfur compounds was considered to be absolutely necessary. The sulfur compounds, which were foreign to the system for the preparation of hydrogen cyanide, are added at the outset in a gaseous form to the initial mixture and must be intro duced constantly during the reaction. As a consequence, the hydrogen cyanide product is contaminated with sulfur compounds.
In spite of these efforts to reduce the performance drop of Al.sub.2 O.sub.3 as a catalyst in the preparation of prussic acid, the working times of the catalyst were at the most 10 days - that is, Al.sub.2 O.sub.3 did not appear to be suitable for a continuous operation.